<h2><html><head><!-- Geotiff converted by Niles --></head><body><tt> </tt> <a href="geotiffhome.html"><img src="gifs/geotiff.gif" alt="GeoTIFF Web Page"></a> <a href="contents.html"><img src="gifs/table.gif" alt="Table of Contents"></a> <a href="geotiff2.html"><img src="gifs/sec2.gif" alt="Top of Section 2"></a></h2>
<img src="gifs/clrbar_half.gif">
 <h3><a name="2.7">2.7 Geocoding Raster Data</a></h3>
<tt> </tt>
<img src="gifs/clrbar_half.gif">
 <h4><a name="2.7.1">2.7.1 General Approach</a></h4>
<tt>A geocoded image is a georeferenced image as described in  <a href="geotiff2.6.html#2.6">section 2.6</a>,
which also specifies a model space coordinate system (CS) between the model
space M (to which the raster space has been tied) and the earth. The
relationship can be diagrammed, including the associated TIFF tags, as
follows:</tt>
<pre>        ModelPixelScaleTag 
        ModelTiepointTag                  GeoKeyDirectoryTag CS 
    R  -------- OR ---------------&gt; M  --------- AND  -----------&gt; Earth
        ModelTransformationTag            GeoDoubleParamsTag  
                                          GeoAsciiParamsTag  </pre><tt><p>
The geocoding coordinate system is defined by the GeoKeyDirectoryTag, while the
Georeferencing information (T) is defined by the ModelTiepointTag and the
ModelPixelScale, or ModelTransformationTag. Since these two systems are
independent of each other, the tags used to store the parameters are separated
from each other in the GeoTIFF file to emphasize the orthogonality. <p>
<p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4><a name="2.7.2">2.7.2 GeoTIFF GeoKeys for Geocoding</a></h4>
<tt>As mentioned above, all information regarding the Model Coordinate System
used in the raster data is referenced from the GeoKeyDirectoryTag, which stores
all of the GeoKey entries. In the Appendix,  <a href="geotiff6.html#6.2">section 6.2</a> summarizes all of the
GeoKeys defined for baseline GeoTIFF, and their corresponding codes are
documented in  <a href="geotiff6.html#6.3.">section 6.3.</a> Only the Keys themselves are documented here.<p>
<p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4>Common Features</h4>
<tt> <p>
     </tt>
<h4>
Public and Private Key and Code Ranges</h4>
<tt><p>
GeoTIFF GeoKey ID's may take any value between 0 and 65535. Following TIFF
general approach, the GeoKey ID's from 32768 and above are available for
private implementations. However, no registry will be established for these
keys or codes, so developers are warned to use them at their own risk.<p>
<p>
The Key ID's from 0 to 32767 are reserved for use by the official GeoTIFF spec,
and are broken down into the following sub-domains:<p>
</tt>
<pre>   [    0,  1023]       Reserved
   [ 1024,  2047]       GeoTIFF Configuration Keys
   [ 2048,  3071]       Geographic/Geocentric CS Parameter Keys
   [ 3072,  4095]       Projected CS Parameter Keys
   [ 4096,  5119]       Vertical CS Parameter Keys
   [ 5120, 32767]       Reserved
   [32768, 65535]       Private use</pre><tt><p>
GeoKey codes, like keys and tags, also range from 0 to 65535. Following the
TIFF approach, all codes from 32768 and above are available for private user
implementation. There will be no registry for these codes, however, and so
developers must be sure that these tags will only be used internally. Use
private codes at your own risk.<p>
<p>
The codes from 0 to 32767 for all public GeoKeys are reserved by this GeoTIFF
specification.<p>
</tt>
<h4>
Common Public Code Values</h4>
<tt><p>
For consistency, several key codes have the same meaning in all implemented
GeoKeys possessing a SHORT numerical coding system:<p>
</tt>
<pre>          0 = undefined
      32767 = user-defined</pre><tt><p>
The "undefined" code means that this parameter is intentionally omitted, for
whatever reason. For example, the datum used for a given map may be unknown, or
the accuracy of a aerial photo is so low that to specify a particular datum
would imply a higher accuracy than is in the data.<p>
<p>
The "user-defined" code means that a feature is not among the standard list,
and is being explicitly defined. In cases where this is meaningful, Geokey
parameters have been supplied for the user to define this feature.<p>
<p>
"User-Defined" requirements: In each section below a specification of the
additional GeoKeys required for the "user-defined" option is given. In all
cases the corresponding "Citation" key is strongly recommended, as per the FGDC
Metadata standard regarding "local" types.<p>
<p>
<p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4>GeoTIFF Configuration GeoKeys</h4>
<tt> <p>
<p>
These keys are to be used to establish the general configuration of this file's
coordinate system, including the types of raster coordinate systems, model
coordinate systems, and citations if any.<p>
<p>
<img src="gifs/clrbar.gif"></tt>
<h4>
GTModelTypeGeoKey      </h4>
<pre>Key ID = 1024
Type: SHORT (code)
Values:  <a href="geotiff6.html#6.3.1.1">Section 6.3.1.1</a> Codes</pre><tt><p>
This GeoKey defines the general type of model Coordinate system used, and to
which the raster space will be transformed:unknown, Geocentric (rarely used),
Geographic, Projected Coordinate System, or user-defined. If the coordinate
system is a PCS, then only the PCS code need be specified. If the coordinate
system does not fit into one of the standard registered PCS'S, but it uses one
of the standard projections and datums, then its should be documented as a PCS
model with "user-defined" type, requiring the specification of projection
parameters, etc. </tt>
<pre>
GeoKey requirements for User-Defined Model Type (not advisable):
     GTCitationGeoKey
     
<img src="gifs/clrbar.gif"></pre><h4>
GTRasterTypeGeoKey       </h4>
<pre>Key ID = 1025  
Type =  <a href="geotiff6.html#6.3.1.2">Section 6.3.1.2</a> codes
</pre><tt>This
establishes the Raster Space coordinate system used; there are currently only
two, namely RasterPixelIsPoint and RasterPixelIsArea. No user-defined raster
spaces are currently supported. For variance in imaging display parameters,
such as pixel aspect-ratios, use the standard TIFF 6.0 device-space tags
instead.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><h4>
GTCitationGeoKey       </h4>
<pre>Key ID = 1026  
Type = ASCII
</pre><tt>As
with all the "Citation" GeoKeys, this is provided to give an ASCII reference to
published documentation on the overall configuration of this GeoTIFF file.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><tt> </tt>
<img src="gifs/clrbar_half.gif">
 <h4>Geographic CS Parameter GeoKeys</h4>
<tt> </tt>
<pre><img src="gifs/clrbar.gif">
</pre><tt>In
general, the geographic coordinate system used will be implied by the projected
coordinate system code. If however, this is a user-defined PCS, or the
ModelType was chosen to be Geographic, then the system must be explicitly
defined here, using the Horizontal datum code.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><h4>
GeographicTypeGeoKey    </h4>
<pre>Key ID = 2048  
Type = SHORT (code)
Values =  <a href="geotiff6.html#6.3.2.1">Section 6.3.2.1</a> Codes    
</pre><tt>This
key may be used to specify the code for the geographic coordinate system used
to map lat-long to a specific ellipsoid over the earth.</tt>
<pre>
GeoKey Requirements for User-Defined geographic CS:
 
       GeogCitationGeoKey
       GeogGeodeticDatumGeoKey
 	GeogAngularUnitsGeoKey (if not degrees)
	GeogPrimeMeridianGeoKey (if not Greenwich)
<img src="gifs/clrbar.gif"></pre><h4>
GeogCitationGeoKey</h4>
<pre>Key ID = 2049
Type = ASCII
Values = text
</pre><tt>General
citation and reference for all Geographic CS parameters.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
GeogGeodeticDatumGeoKey    </h4>
<pre>Key ID = 2050  
Type = SHORT (code)
Values =  <a href="geotiff6.html#6.3.2.2">Section 6.3.2.2</a> Codes    
</pre><tt>This
key may be used to specify the horizontal datum, defining the size, position
and orientation of the reference ellipsoid used in user-defined geographic
coordinate systems.</tt>
<pre>
GeoKey Requirements for User-Defined Horizontal Datum: 
       GeogCitationGeoKey
       GeogEllipsoidGeoKey
<img src="gifs/clrbar.gif"></pre><h4>
GeogPrimeMeridianGeoKey</h4>
<pre>Key ID = 2051
Type = SHORT (code)
Units:  <a href="geotiff6.html#6.3.2.4">Section 6.3.2.4</a> code
</pre><tt>Allows
specification of the location of the Prime meridian for user-defined geographic
coordinate systems. The default standard is Greenwich, England.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
GeogPrimeMeridianLongGeoKey</h4>
<pre>Key ID = 2061  
Type = DOUBLE 
Units =  GeogAngularUnits</pre><tt></tt>
<pre>This key allows definition of user-defined Prime Meridians, the location of which is defined by its longitude relative to Greenwich.
<img src="gifs/clrbar.gif"></pre><h4>
GeogLinearUnitsGeoKey </h4>
<pre>Key ID = 2052
Type = SHORT
Values:  <a href="geotiff6.html#6.3.1.3">Section 6.3.1.3</a> Codes
</pre><tt>Allows
the definition of geocentric CS linear units for user-defined GCS.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><h4>
GeogLinearUnitSizeGeoKey </h4>
<pre>Key ID = 2053
Type = DOUBLE
Units: meters
</pre><tt>Allows
the definition of user-defined linear geocentric units, as measured in
meters.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
GeogAngularUnitsGeoKey</h4>
<pre>Key ID = 2054  
Type = SHORT (code)
Values =   <a href="geotiff6.html#6.3.1.4">Section 6.3.1.4</a>  Codes 
</pre><tt>Allows
the definition of <b>geocentric</b> CS Linear units for user-defined GCS and
for ellipsoids.</tt>
<pre>
GeoKey Requirements for "user-defined" units:
    GeogCitationGeoKey
    GeogAngularUnitSizeGeoKey 
<img src="gifs/clrbar.gif"></pre><h4>
GeogAngularUnitSizeGeoKey </h4>
<pre>Key ID = 2055
Type = DOUBLE
Units: radians
</pre><tt>Allows
the definition of user-defined angular geographic units, as measured in
radians.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
GeogEllipsoidGeoKey</h4>
<pre>Key ID = 2056
Type = SHORT (code)
Values =  <a href="geotiff6.html#6.3.2.3">Section 6.3.2.3</a> Codes 
   </pre><tt>This
key may be used to specify the coded ellipsoid used in the geodetic datum of
the Geographic Coordinate System. </tt>
<pre>
GeoKey Requirements for User-Defined Ellipsoid: 
   GeogCitationGeoKey
   [GeogSemiMajorAxisGeoKey, 
           [GeogSemiMinorAxisGeoKey | GeogInvFlatteningGeoKey] ]
<img src="gifs/clrbar.gif"></pre><h4>
GeogSemiMajorAxisGeoKey  </h4>
<pre>Key ID = 2057
Type = DOUBLE
Units: Geocentric CS Linear Units
</pre><tt>Allows
the specification of user-defined Ellipsoid Semi-Major Axis (a). </tt>
<pre>
<img src="gifs/clrbar.gif"></pre><h4>
GeogSemiMinorAxisGeoKey  </h4>
<pre>Key ID = 2058
Type = DOUBLE
Units: Geocentric CS Linear Units
</pre><tt>Allows
the specification of user-defined Ellipsoid Semi-Minor Axis (b). </tt>
<pre>
<img src="gifs/clrbar.gif"></pre><h4>
GeogInvFlatteningGeoKey</h4>
<pre>Key ID = 2059
Type = DOUBLE
Units: none.
</pre><tt>Allows
the specification of the <b>inverse</b> of user-defined Ellipsoid's flattening
parameter (f). The eccentricity-squared e^2 of the ellipsoid is related to the
non-inverted f by:</tt>
<pre>
      e^2  = 2*f  - f^2
 
   Note: if the ellipsoid is spherical the inverse-flattening
   becomes infinite; use the GeogSemiMinorAxisGeoKey instead, and
   set it equal to the semi-major axis length.
<img src="gifs/clrbar.gif"></pre><h4>
GeogAzimuthUnitsGeoKey</h4>
<pre>Key ID = 2060  
Type = SHORT (code)
Values =   <a href="geotiff6.html#6.3.1.4">Section 6.3.1.4</a> Codes 
</pre><tt>This
key may be used to specify the angular units of measurement used to defining
azimuths, in geographic coordinate systems. These may be used for defining
azimuthal parameters for some projection algorithms, and may not necessarily be
the same angular units used for lat-long.</tt>
<pre></pre><tt></tt>
<pre><img src="gifs/clrbar.gif"></pre><tt><p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4>Projected CS Parameter GeoKeys</h4>
<tt> <p>
<p>
The PCS range of GeoKeys includes the projection and coordinate transformation
keys as well. The projection keys are included in this block since they can
only be used to define projected coordinate systems.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjectedCSTypeGeoKey   </h4>
<pre>Key ID = 3072
Type = SHORT (codes)
Values:  <a href="geotiff6.html#6.3.3.1">Section 6.3.3.1</a> codes</pre><tt><p>
This code is provided to specify the projected coordinate system.</tt>
<pre>
GeoKey requirements for "user-defined" PCS families:
   PCSCitationGeoKey
   ProjectionGeoKey
 <img src="gifs/clrbar.gif"></pre><h4>
PCSCitationGeoKey</h4>
<pre>Key ID = 3073
Type = ASCII
</pre><tt>As
with all the "Citation" GeoKeys, this is provided to give an ASCII reference to
published documentation on the Projected  Coordinate System particularly if
this is a "user-defined" PCS.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><tt><p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4>Projection Definition GeoKeys</h4>
<tt> </tt>
<pre><img src="gifs/clrbar.gif">
</pre><tt>With
the exception of the first two keys, these are mostly  projection-specific
parameters, and only a few will be required for any particular projection type.
Projected coordinate systems automatically imply a specific projection type, as
well as specific parameters for that projection, and so the keys below will
only be necessary for user-defined projected coordinate systems.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjectionGeoKey</h4>
<pre>Key ID = 3074
Type = SHORT (code)
Values:   <a href="geotiff6.html#6.3.3.2">Section 6.3.3.2</a> codes
</pre><tt>Allows
specification of the coordinate transformation method and projection zone
parameters.  Note : when associated with an appropriate Geographic Coordinate
System, this forms a Projected Coordinate System.<p>
</tt>
<pre>
GeoKeys Required for "user-defined" Projections:</pre><tt></tt>
<pre>   PCSCitationGeoKey
   ProjCoordTransGeoKey
   ProjLinearUnitsGeoKey
   (additional parameters depending on ProjCoordTransGeoKey).
<img src="gifs/clrbar.gif"></pre><h4>
ProjCoordTransGeoKey</h4>
<pre>Key ID = 3075
Type = SHORT (code)
Values:   <a href="geotiff6.html#6.3.3.3">Section 6.3.3.3</a> codes
</pre><tt>Allows
specification of the coordinate transformation method used. Note: this does not
include the definition of the corresponding Geographic Coordinate System to
which the projected CS is related; only the transformation method is defined
here.</tt>
<pre>
GeoKeys Required for "user-defined" Coordinate Transformations:
   PCSCitationGeoKey
   &lt;additional parameter geokeys depending on the Coord. Trans. specified).
<img src="gifs/clrbar.gif"></pre><h4>
ProjLinearUnitsGeoKey</h4>
<pre>Key ID = 3076 
Type = SHORT (code)
Values:  <a href="geotiff6.html#6.3.1.3">Section 6.3.1.3</a> codes 
</pre><tt>Defines
linear units used by this projection.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjLinearUnitSizeGeoKey</h4>
<pre>Key ID = 3077 
Type = DOUBLE
Units: meters
</pre><tt>Defines
size of user-defined linear units in meters.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjStdParallel1GeoKey</h4>
<pre>Key ID = 3078 
Type = DOUBLE
Units: GeogAngularUnit 
Alias: ProjStdParallelGeoKey (from Rev 0.2)
</pre><tt>Latitude
of primary Standard Parallel.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjStdParallel2GeoKey</h4>
<pre>Key ID = 3079
Type = DOUBLE
Units: GeogAngularUnit 
</pre><tt>Latitude
of second Standard Parallel.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjNatOriginLongGeoKey</h4>
<pre>Key ID = 3080
Type = DOUBLE
Units: GeogAngularUnit 
Alias: ProjOriginLongGeoKey
</pre><tt>Longitude
of map-projection Natural origin. </tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjNatOriginLatGeoKey</h4>
<pre>Key ID = 3081
Type = DOUBLE
Units: GeogAngularUnit
Alias: ProjOriginLatGeoKey
</pre><tt>Latitude
of map-projection Natural origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseEastingGeoKey</h4>
<pre>Key ID = 3082
Type = DOUBLE
Units: ProjLinearUnit </pre><tt><p>
Gives the easting coordinate of the map projection Natural origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseNorthingGeoKey</h4>
<pre>Key ID = 3083
Type = DOUBLE
Units: ProjLinearUnit </pre><tt><p>
Gives the northing coordinate of the map projection Natural origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseOriginLongGeoKey</h4>
<pre>Key ID = 3084
Type = DOUBLE
Units: GeogAngularUnit </pre><tt><p>
Gives the longitude of the False origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseOriginLatGeoKey</h4>
<pre>Key ID = 3085
Type = DOUBLE
Units: GeogAngularUnit </pre><tt><p>
Gives the latitude of the False origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseOriginEastingGeoKey</h4>
<pre>Key ID = 3086
Type = DOUBLE
Units: ProjLinearUnit </pre><tt><p>
Gives the easting coordinate of the false origin. This is NOT the False
Easting, which is the easting attached to the Natural origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseOriginNorthingGeoKey</h4>
<pre>Key ID = 3087
Type = DOUBLE
Units: ProjLinearUnit </pre><tt><p>
Gives the northing coordinate of the False origin. This is NOT the False
Northing, which is the northing attached to the Natural origin.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjCenterLongGeoKey</h4>
<pre>Key ID = 3088
Type = DOUBLE
Units: GeogAngularUnit 
</pre><tt>Longitude
of Center of Projection. Note that this is not necessarily the origin of the
projection.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjCenterLatGeoKey</h4>
<pre>Key ID = 3089
Type = DOUBLE
Units: GeogAngularUnit
</pre><tt>Latitude
of Center of Projection. Note that this is not necessarily the origin of the
projection.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjCenterEastingGeoKey</h4>
<pre>Key ID = 3090
Type = DOUBLE
Units: ProjLinearUnit </pre><tt><p>
Gives the easting coordinate of the center. This is NOT the False Easting.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjFalseOriginNorthingGeoKey</h4>
<pre>Key ID = 3091
Type = DOUBLE
Units: ProjLinearUnit </pre><tt><p>
Gives the northing coordinate of the center. This is NOT the False
Northing.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjScaleAtNatOriginGeoKey</h4>
<pre>Key ID = 3092
Type = DOUBLE
Units: none
Alias: ProjScaleAtOriginGeoKey (Rev. 0.2)
</pre><tt>Scale
at Natural Origin. This is a ratio, so no units are required.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjScaleAtCenterGeoKey</h4>
<pre>Key ID = 3093 
Type = DOUBLE
Units: none
</pre><tt>Scale
at Center. This is a ratio, so no units are required.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjAzimuthAngleGeoKey</h4>
<pre>Key ID = 3094
Type = DOUBLE
Units: GeogAzimuthUnit
 </pre><tt>Azimuth
angle east of true north of the central line passing through the projection
center (for elliptical (Hotine) Oblique Mercator). Note that this is the
standard method of measuring azimuth, but is opposite the usual mathematical
convention of positive indicating counter-clockwise.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
ProjStraightVertPoleLongGeoKey   </h4>
<pre>Key ID = 3095 
Type = DOUBLE
Units: GeogAngularUnit 
</pre><tt>Longitude
at Straight Vertical Pole. For polar stereographic.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
GeogAzimuthUnitsGeoKey</h4>
<pre>Key ID = 2060  
Type = SHORT (code)
Values =   <a href="geotiff6.html#6.3.1.4">Section 6.3.1.4</a> Codes 
</pre><tt>This
key is actually part of the "Geographic CS Parameter Keys" section, but is
mentioned here as it is useful for defining units used in the azimuthal
projection parameters.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><tt><p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4>Vertical CS Parameter Keys</h4>
<tt> <p>
<p>
Note: Vertical coordinate systems are not yet implemented. These sections are
provided for future development, and any vertical coordinate systems in the
current revision must be defined using the VerticalCitationGeoKey.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
VerticalCSTypeGeoKey</h4>
<pre>Key ID = 4096  
Type = SHORT (code)
Values =   <a href="geotiff6.html#6.3.4.1">Section 6.3.4.1</a>  Codes 
</pre><tt>This
key may be used to specify the vertical coordinate system.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
VerticalCitationGeoKey</h4>
<pre>Key ID = 4097
Type = ASCII
Values =  text 
</pre><tt>This
key may be used to document the vertical coordinate system used, and its
parameters.</tt>
<pre><img src="gifs/clrbar.gif"></pre><h4>
VerticalDatumGeoKey</h4>
<pre>Key ID = 4098   
Type = SHORT (code)
Values =   <a href="geotiff6.html#6.3.4.2">Section 6.3.4.2</a>  codes
</pre><tt>This
key may be used to specify the vertical datum for the vertical coordinate
system.</tt>
<pre>
<img src="gifs/clrbar.gif"></pre><h4>
VerticalUnitsGeoKey</h4>
<pre>Key ID = 4099   
Type = SHORT (code)
Values =   <a href="geotiff6.html#6.3.1.3">Section 6.3.1.3</a>  Codes 
</pre><tt>This
key may be used to specify the vertical units of measurement used in the
geographic coordinate system, in cases where geographic CS's need to reference
the vertical coordinate. This, together with the Citation key, comprise the
only fully implemented keys in this section, at present.<p>
<p>
<p>
<p>
 </tt>
<img src="gifs/clrbar_half.gif">
 <h4><a name="2.7.3">2.7.3 Cookbook for Geocoding Data</a></h4>
<pre>
Step 1: Determine the Coordinate system type of the raster data, based on
        the nature of the data: pixels derived from scanners or other 
        optical devices represent areas, and most commonly will use the
        RasterPixelIsArea coordinate system. Pixel data such as digital
        elevation models represent points, and will probably use 
        RasterPixelIsPoint coordinates.
           Store in: GTRasterTypeGeoKey
Step 2: Determine which class of model space coordinates are most natural
        for this dataset:Geographic, Geocentric, or Projected Coordinate 
        System. Usually this will be PCS.
        
           Store in: GTModelTypeGeoKey
           
Step 3: This step depends on the GTModelType:
      case PCS:  Determine the PCS projection system. Most of the
           PCS's used in standard State Plane and national grid systems
           are defined, so check this list first; the EPSG index in 
            <a href="geotiff6.html#6.4">section 6.4</a> may be useful for this purpose.
               
           Store in: ProjectedCSTypeGeoKey, ProjectedCSTypeGeoKey
           If coded, it will not be necessary to specify the Projection 
           datum, etc for this case, since all of those parameters
           are determined by the ProjectedCSTypeGeoKey code. Skip to
           step 4 from here.
               
           If none of the coded PCS's match your system, then this is a
           user-defined PCS. Use the Projection code list to check for
           standard projection systems.
           Store in: ProjectionGeoKey and skip to Geographic CS case.
           If none of the Projection codes match your system, then this
           is a user-defined projection. Use the ProjCoordTransGeoKey to
           specify the coordinate transformation method (e.g. Transverse
           Mercator), and all of the associated parameters of that method.
           Also define the linear units used in the planar coordinate
           system.
           
           Store in: ProjCoordTransGeoKey, ProjLinearUnitsGeoKey  
               &lt;and other CT related parameter keys&gt;
           Now continue on to define the Geographic CS, below.
      case GEOCENTRIC:
      case GEOGRAPHIC:  Check the list of standard GCS's and use the
           corresponding code. To use a code both the Datum, Prime
           Meridian, and angular units must match those of the code.
                
           Store in:  GeographicTypeGeoKey and skip to Step 4.
           If none of the coded GCS's match exactly, then this is a
           user-defined GCS. Check the list of standard datums,
           Prime Meridians, and angular units to define your system.
           Store in: GeogGeodeticDatumGeoKey, GeogAngularUnitsGeoKey,
              GeogPrimeMeridianGeoKey and skip to Step 4. 
  
           If none of the datums match your system, you have a 
           user-defined datum, which is an odd system, indeed. Use 
           the GeogEllipsoidGeoKey to select the appropriate ellipsoid
           or use the GeogSemiMajorAxisGeoKey, GeogInvFlatteningGeoKey to 
           define, and give a reference using the GeogCitationGeoKey.
           Store in: GeogEllipsoidGeoKey, etc. and go to Step 4.
        
Step 4: Install the GeoKeys/codes into the GeoKeyDirectoryTag, and the
      DOUBLE and ASCII key values into the corresponding value-tags.
      
Step 5: Having completely defined the Raster &amp; Model coordinate system,
      go to Cookbook  <a href="geotiff2.6.html#2.6.2">section 2.6.2</a> and use the Georeferencing Tags
      to tie the raster image down onto the Model space.
</pre><tt> </tt>
</body></html>
